Pasta, along with bread, is the cornerstone of an endurance athlete’s diet. But should it be?
Photograph: Antonio Calanni/AP

In the mid 1960s, a group of Swedish physiologists based in Stockholm conducted a landmark experiment that would come to dominate the way every athlete, from Olympians to club runners, thought about the relationship between diet and performance for the rest of the 20th century.

Around 130 years earlier, the French anatomist Guillaume-Benjamin-Amand Duchenne de Boulogne had developed a technique called muscle biopsy, a surgical method for extracting tiny pieces of tissue from patients for further examination. Duchenne was interested in discovering the causes of muscular dystrophy, a degenerative muscle-weakening disease, the most severe form of which affects around 1 in 3,600 boys, typically leaving them wheelchair dependent by the age of 12.

The Swedish team was to use the first to use muscle biopsy in the field of sports performance. The researchers were interested in whether there was a link between the amount of glycogen stored in muscle fibres and the speed at which an athlete could cover a certain distance. In 1967, the results were announced. They found that by increasing carbohydrate intake, glycogen levels could also be increased. In short, athletes could run both further and faster.

The study made instant headlines around the world, and an entire new field was born. “This was seen as one of the first great breakthroughs in sports science,” says Professor Tim Noakes of the University of Cape Town. “We now believed we knew exactly what determined your running performance in marathons – it was how much glycogen you stored before the race. And that gave us power as scientists. We were able to tell people to ‘carb-load’, and we felt very important doing that.”

Such carbohydrate-loading programs typically recommend eating around 500-700g a day, around 7-12g per kg of body weight. While unnecessary for 10K runs, as your body naturally contains enough glycogen to last the course, many runners will struggle to complete with the half marathon distance or above without additional fuel. For elite distance runners, the moment when they have exhausted their body’s existing supply of glycogen comes somewhere between the half and full marathon mark.

In running circles, our belief in the benefits of carbs has been effectively endorsed by the overwhelming success of east African distance runners, almost all of whom exist on a basic, starch-rich diet consisting of rice, pasta, lentils, porridge and vegetables.

But not everyone believes the story is quite this simple.

Over the past decade, a group of scientists has been challenging the status quo. They claim the optimal fuel for exercise is not high levels of carbohydrate but fat, a nutritional balance achieved through diets such as Banting or New Atkins. The body naturally contains far greater reserves of fat, which some see as an indicator that this is really the preferred energy source. After six weeks of being on a high-fat diet, the body switches to burning fatty ketones, instead of the sugars in carbohydrate and, with such a larger supply of fuel, it’s possible to cover the full marathon distance or even an ultra run, without running out of energy.

So who is right? Professor Louise Burke, head of sports nutrition at the Australian Institute for Sport, says: “A range of options in the dietary tool box is likely to be a better model for optimal sports nutrition than insisting on a single, one-size-fits-all solution.” Essentially, the actual answer may come down to your own individual genetics – and also, perhaps, your age.

While Ethiopian and Kenyan runners excel on a high carbohydrate intake, not everyone has the same metabolic make-up. A leading voice in the health and fitness world in South Africa for many years, Noakes was once an advocate of carb loading until personal experience made him question the wisdom of what he was preaching. A keen marathon and ultra-marathon runner, he was shocked to find he had developed type-2 diabetes.

“I thought exercising was making me healthier but what I didn’t realise was that my diet contained too much carbohydrate for my own physiology, and this was negating all the benefits,” he says.

Noakes had reached the state where the body secretes such high levels of the hormone insulin in order to try to process all the sugars in the carbohydrate being consumed that it becomes resistant to it. Prolonged insulin resistance leads to an overload of the pancreatic cells which produce insulin, resulting in drastic cell death and diabetes.

But the quantities of carbohydrate you have to eat to develop insulin resistance vary hugely from person to person. “People who are carbohydrate-sensitive, and can metabolise it very quickly, will benefit more from a high carb diet,” Noakes says. “The Kenyan runners, who are all extremely lean, definitely fall into this category. I get reports from individual runners who say they can’t perform anything like as well on a high-fat diet. But if you’re insulin-resistant or prone to developing this condition, then you’ll really benefit from a high-fat regime.”

As we get older, there are suggestions that the body’s metabolic machinery begins to wear slightly thin, indicating that we should perhaps adapt by reducing our carbohydrate intake in turn. But many runners fear that by switching to high fat they will cut their speed.

Competitors such as Iron Man triathletes and Grand Tour cyclists, who require endurance most of all, particularly benefit from low-carb, high-fat diets, as they can tap into a far deeper reserve of energy. Many professional cyclists consume just 150g of carbohydrate a day.

But for half-marathons and marathons, the evidence so far suggests that high-fat runners could be slightly slower. There may be a limit to the rate at which ketones can be burned and supplied to the muscles to provide the energy needed to sustain quicker paces. However, Noakes says that this is only really noticeable if you are at a truly elite level. “If you’re going to run a marathon under two hours and 10 minutes, then yes, you probably need lots of carbohydrate to be able to perform to such an intense level of activity, but once you’re slower than two and a half hours, I’m not really convinced you do.”

One compromise that has been gaining increasing popularity in recent years is the “train low, race high” regime favoured by runners ranging from Britain’s Steve Way to Meb Keflezighi, silver medallist in the marathon at the Athens Olympics.

“The idea is that by training on a low-carbohydrate, high-fat diet you force your body to shift some of your fuel use to fat so you become more efficient,” says Steve Magness who trains elite cross-country runners at the University of Houston. “Initially, when you’re not fat adapted, it will increase the stress on your body. But just as weightlifters get stronger by steadily increasing the weight of the dumbbells, the production of mitochondria in your muscles increases and you become better aerobically. Then, the day before the race, you fuel up with carbohydrate, so that you’ll also be able to get the rapid energy supply you need to sustain that really high pace through 26.2 miles.”

But is there still a limit to the amount of carbohydrate you should take in any one day, even when loading up prior to a marathon? Noakes believes it’s pointless to consume in excess of 300g/day as the body simply isn’t cut out to store any more than that. “There is a neurological phenomenon to getting carbohydrate in the short term. It motivates you and makes you feel good. But if you eat too much, you can’t store it in your muscles while exercising and so you end up producing all this insulin to burn the excess later in the day when you’re resting and you don’t need it anymore.”